Conventional humidity sensors are designed to detect humidity through changes of electrical properties, typically electric resistance. Known sensors use electrolytes such as lithium chloride, metal oxides, and organic polymers as the humidity sensitive material. However, the humidity sensors using electrolytes such as lithium chloride can measure only a narrow range of humidity and are less resistant to water in that their performance can be altered by dew condensation and wetting. The humidity sensors using metal oxides are resistant to water, but low sensitive. Because of the lack of long-term stability when used alone, they require a heat cleaning circuit which would add to the operating cost. The sensor structure is complex.
Among the humidity sensitive materials, organic polymers, especially polymeric electrolytes having a quaternary ammonium salt group have been widely used in commercial and industrial applications and so appreciated. For example, Japanese Patent Publication (JP-B) No. 54176/1986 discloses a humidity sensitive material comprising aggregates of latex particles formed of a copolymer between a hydrophobic monomer and an ionic or non-ionic hydrophilic monomer and having a hydrophilic surface layer. There are exemplified some cationic compounds having primary to quaternary ammonium salts.
JP-B 7976/1987 discloses a humidity sensitive material in the form of a polymer which is obtained by polymerizing 2-hydroxy-3-methacryloxypropyltrimethylammonium chloride to a degree of polymerization of 1,000 to 10,000.
JP-B 24465/1990 discloses a humidity sensitive thin film of a polymer having the structural formula: EQU --(N.sup.+ (R.sup.1)(R.sup.2)X.sup.- --A--N.sup.+ (R.sup.3)(R.sup.4)X.sup.---B).sub.n --
wherein R.sup.1 to R.sup.4 are alkyl, X.sup.- is a halide ion, A and B are --(CH.sub.2).sub.m -- wherein m.gtoreq.2. The polymer may be blended with another polymer such as polyvinyl pyrrolidone for the purposes of improving adhesion to a substrate and water resistance. The blend is also effective in forming a humidity sensitive thin film.
Humidity sensors using the polymeric electrolytes exemplified above as the humidity sensitive material, however, are still low in water resistance in that the polymeric electrolytes can be partially leached in a high humidity region, especially in a dew condensing atmosphere. They also suffer from a hysteresis phenomenon that they produce different outputs at the same humidity depending on whether the humidity is increasing or decreasing. In a low humidity region having a relative humidity (RH) of less than 10%, they have so high resistance values that practical humidity measurement is impossible.